Chlorine dioxide (ClO2) is an oxidizing agent used to sterilize drinking water and as a bactericide in general water treatment. Major uses of chlorine dioxide today include disinfection, control of taste and odor, control of iron and manganese, and control of hydrogen sulfide and phenolic compounds. The metabolism of microorganisms and consequently their ability to survive and propagate are influenced by the oxidation reduction potential (ORP) of the medium in which they live. Chlorine dioxide is a neutral compound of chlorine in the +IV oxidation state. It disinfects by oxidation; however, it does not chlorinate. It is a relatively small, volatile, and highly energetic molecule, and a free radical even while in dilute aqueous solutions. At high concentrations, it reacts violently with reducing agents. However, it is stable in dilute solution in a closed container in the absence of light. Chlorine dioxide functions as a highly selective oxidant due to its unique, one-electron transfer mechanism where it is reduced to chlorite. The pKa for the chlorite ion, chlorous acid equilibrium, is extremely low at pH 1.8. The oxidation reduction of some key reactions are:ClO2(aq)+e−=ClO2— Eo=0.954V
Other important half reactions are:ClO2—+2H2O+4e−=Cl—+4OH— Eo=0.76VClO3—+H2O+2e−=ClO2—+2OH— Eo=0.33VClO3—+2H++e−=ClO2+H2O Eo=1.152V
One of the known methods for producing chlorine dioxide uses a mixture of sodium chlorate, an acid and hydrogen peroxide with the hydrogen peroxide functioning as a reducing agent in the reaction. The use of hydrogen peroxide in the chlorine dioxide generation process, however, releases oxygen gas as a by-product of the reaction. The generation and release of oxygen gas presents an engineering problem for the design, construction and operation of chlorine dioxide generators utilizing this chemistry. In particular, the generator must be configured for safely accommodating the generation and release of oxygen without resulting in excessive pressures or accumulations increases the risk of equipment failures and/or explosions.
Hydrogen peroxide use in chlorine dioxide generation chemistries has been detailed in a number of patents and other publications including, for example: Yin, G., “Mechanism of the ClO2 Generation from the H2O2—HClO3 Reaction,” Can. J. Chem. Eng., 78(2000) 827-33; U.S. Pat. Nos. 5,091,167; 5,366,714; 5,380,517; 5,565,182; 5,895,638; 6,576,213; 6,790,427 and 7,070,710 and European Patent No. 1198411, the contents of which are incorporated herein, in their entirety, by reference.
Another known method for producing chlorine dioxide uses a mixture of sodium chlorate, an acid and methanol with the methanol functioning as a reducing agent in the reaction. However, this method can be costly and inefficient in terms of chloride dioxide production and involves the handling of methanol which is flammable. Disclosures relating to methanol use in chlorine dioxide generation chemistries have been detailed in a number of patents and other publications including, for example, Fredette, M. C., “Bleaching Chemicals: Chlorine Dioxide,” Pulp Bleaching—Principles and Practice, Chap. 2, TAPPI Press (1996); Ni, Y., et al., “Mechanism of the Methanol-Based ClO2 Generation Process,” J. Pulp Paper Sci. 23, J346-52 (1997); Ni, Y., et al., “Mechanism of the Methanol-Based ClO2 Generation Process,” 1996 International Pulp Bleaching Conference; Brown, C. J., et al., “Ion-exchange technologies for the minimum effluent kraft mill,” CPPA Technical Section Symposium on System Closure II, Montreal, Jan. 26-30, 1998, and U.S. Pat. Nos. 4,081,520; 4,473,540; 5,116,595; 5,674,466 and 5,770,171, the contents of which are incorporated herein, in their entirety, by reference.
These and other issues are addressed by the present disclosure. It is an object of this disclosure to provide novel systems and methods for chlorine dioxide production using an unexpectedly beneficial reducing agent in the form of a phosphine. Advantages of the present disclosure over existing methods for chlorine dioxide production include not producing oxygen as a byproduct, no flammable components are required, and efficient chlorine dioxide production is achieved.